The present invention relates to surveillance satellites, and more particularly, to a system and antenna configuration for a multiple beam radar surveillance satellite, particularly suitable for being operated in medium earth orbits (MEO).
Various radar surveillance systems have been proposed using low Earth orbit satellites. One drawback to low earth orbit satellites is that a large constellation is required to provide continuous surveillance coverage. Large low Earth orbit constellations may include dozens of satellites to provide adequate surveillance over a given region of interest. A significant drawback to providing such number of satellites is cost and complexity. To launch and maintain such a system may be cost prohibitive. Also, additional sensor and communications capabilities, due to cost reasons, may be eliminated from the satellite to attempt to reduce these high costs. However, by reducing these features, the flexibility and overall surveillance utility of the system is reduced. Ideally, the problem of persistent surveillance over a region of interest could be accommodated by a few satellites in geostationary orbit. However, the size of the antenna and amount of transmitter power are strong functions of distance to the target being detected and therefore make satellites at geostationary orbits prohibitively large. This is undesirable in many circumstances.
It is desirable that a radar surveillance satellite be able to surveill and track targets almost simultaneously over a wide area within the satellites instantaneous field of regard. A way to achieve this capability is to employ an antenna that allows electronic beam steering, so that swaths hundreds of miles apart may be surveilled almost simultaneously (within a few seconds), and targets hundreds of miles apart may be tracked simultaneously by switching a beam of radar energy back and forth between them every few seconds.
From low Earth orbit, hundreds of miles distance on the ground translates into large angles (tens of degrees) at the satellite, so, for electronic beam steering to work, an electronically-steered phased-array antenna must be used. Such antenna are expensive, and compounded with the need for a large number of them required in low Earth orbit, may contribute to the excessive costliness of a low Earth orbit radar surveillance system.
In medium Earth orbit, the number of satellites needed for a radar surveillance system is drastically reduced. Also the angle over which the radar beam must rapidly steer, in order to simultaneously track targets hundreds of miles apart, is also drastically reduced (to a few degrees).
It would therefore be desirable to provide a satellite system having a constellation design that reduces the number of satellites compared to a low Earth orbit satellite system. It would also be desirable to provide a system that reduces the steering angle needed to simultaneously track targets that are hundreds of miles apart.
One object of the invention is to provide a surveillance satellite system having a reduced number of satellites with substantial surveillance coverage.
It is a further object of the invention to provide a system that allows continuous surveillance below 40xc2x0 Latitude and provide less than 1 minute mean time to access below 70xc2x0 Latitude.
In one aspect of the invention, a surveillance system includes a plurality of satellites a plurality of medium earth orbit satellites, each satellite having an antenna having a main reflector and a subreflector which is electromagnetically coupled to the main reflector. A phased array feed generates multiple electromagnetic beams positioned so the electromagnetic beams reflect from the subreflector onto the main reflector which collimates the beam for transmission to the surveilled region on the Earth""s surface.
In a further aspect of the invention, a method of detecting slow moving targets takes advantage of the longer integration times available at the MEO altitude to improve target detectability and reduced false alarm rate. The method comprises the following processing steps which may be performed at the ground station:
establishing a discrete threshold;
obtaining a sum signal;
comparing the sum signal to the discrete threshold;
when the sum signal exceeds the discrete threshold, determining a monopulse anomaly;
when a monopulse anomaly is determined, classifying the signal as a stable mover;
when a monopulse anomaly is not found, statistically determining surrounding cells of an image to determine a power associated with interference or a discrete; when the power ratio is low, the cells are classified as interference or a discrete; when the power ratio is high, classifying as an unstable mover.
One advantage of the invention is that the satellite constellation supports various types of operations including communication network augmentation. If ground stations are located in a location far from the region of interest, the satellites may use the crosslinks to communicate the collected information to the appropriate satellite for downlinking for further processing. Another advantage of the invention is that the satellite payload may also be used passively to surveill electronic emanations from the area being surveilled or operate as a transmitter only to beam radar energy to the surveilled area for cooperative reception by airborne platforms to detect and track smaller targets of interest.